Linear polyesters such as poly(alkylene terephthalate) are generally known and commercially available where the alkylene typically has 2 to 8 carbon atoms. Linear polyesters have many valuable characteristics including strength, toughness, high gloss, and solvent resistance. Linear polyesters are conventionally prepared by the reaction of a diol with a dicarboxylic acid or its functional derivative, typically a diacid halide or ester. Linear polyesters may be fabricated into articles of manufacture by a number of known techniques including extrusion, compression molding, and injection molding.
Recently, macrocyclic oligoesters were developed that have unique properties that make them attractive for a variety of applications, including as matrices for engineering thermoplastic composites. Macrocyclic oligoesters exhibit low melt viscosity, for example, allowing them easily to impregnate a dense fibrous preform followed by polymerization to polymers. Furthermore, certain macrocyclic oligoesters melt and polymerize at temperatures well below the melting point of the resulting polymer. Upon melting and in the presence of an appropriate catalyst, polymerization and crystallization can occur virtually isothermally.
Macrocyclic oligoesters prepared from depolymerzation of polyester linears in the presence of a depolymerizaing catalyst typically do not re-polymerize to form high molecular weight polyesters (HMWPs) because macrocyclic oligoesters prepared by depolymerization contain small amount of acid impurities, i.e., acidic impurities such as carboxylic acid-terminated species. The carboxylic acid-terminated species, e.g., carboxylic acid-terminated monomers, oligomers, and polymers, inhibit macrocyclic oligoesters from polymerizing to HMWPs. Macrocyclic oligoesters are usually prepared from depolymerization of polyester linears. It can be useful, therefore, to remove such carboxylic acid-terminated oligomers to allow formation of HMWPs.
Common acid absorbents, such as basic alumina, carbon, silica or molecular sieves, have been employed to remove acid impurities from macrocyclic oligoesters. See, e.g., U.S. Pat. No. 5,434,244. Generally, protonated molecular sieves have pores sizes that allow polar groups of impurities to be subjected to the protonated sites. A solution of a macrocyclic oligoester may be contacted with the protonated molecular sieves to remove impurities, which are adsorbed by the protonated sites in the molecular sieves. Typically, about 15 g to 100 g of protonated molecular sieves are needed to purify every gram of impurity from macrocyclic oligoesters. Since molecular sieves absorb acid slowly, large columns and slow flow rates are typically needed which makes the cost of using molecular sieves prohibitively high and the production efficiency low. There is not an efficient and effective way to regenerate molecular sieves that must be discarded after use.
Other techniques of removing acid impurities include passing a solution of macrocyclic oligoesters containing acidic species over beds of activated basic alumina. Alumina is effective at acid absorption, but alumina is also expensive and must be discarded after removal of the acidic species from the solution. Use of alumina thus requires both new materials and material disposal. Typically, one part by weight alumina can absorb acid impurities contained in about 10-20 parts of macrocyclic oligoester before fresh alumina is required.